1. Field of the Invention
The present invention relates to a fuel injection method and apparatus for a cylinder direct-injection engine, and more paticularly, to a measure to counter the coincidence of fuel injection timings for a plurality of cylinders during combustion mode switching operation.
2. Information of the Related Art
In a cylinder direct-injection engine for a vehicle, an injector is mounted in a combustion chamber, and a fuel under high pressure is directly injected from the injector into a cylinder in a compression stroke, which starts when a scavenging port is closed and terminates when ignition starts. Combustion in the cylinder has two modes i.e., stratified combustion and homogeneous combustion. The stratified combustion is effected with a relatively late fuel injection timing during idling operation or low- or medium-load operation, while the homogeneous combustion is effected with a relatively early fuel injection timing during high-load operation. When the load is changed from low or medium load to high load, or contrariwise, therefore, the combustion mode is switched from the stratified combustion to the homogeneous combustion, or in the opposite way.
When the combustion mode is switched in this manner, the fuel injection timing is changed. The following is a description of the way the injection timing is changed as the combustion mode is switched, in a two-cycle four-cylinder engine with a firing order of a second cylinder (#2), a first cylinder (#1), a fourth cylinder (#4) and a combustion mode is switched from the stratified combustion to the homogeneous combustion when combustion is being started in a first cylinder (#1) after the stratified combustion is executed in a second cylinder (#2). In this case, the fuel injection timing for the second cylinder is relatively late, since the stratified combustion is executed in this cylinder (#2). For the first cylinder, on the other hand, the fuel injection timing is relatively early, since the homogeneous combustion is executed in this cylinder (#2). Accordingly, the injection timing for the second cylinder may coincide with the injection timing for the first cylinder.
Meanwhile, a pulse for the fuel injection timing is generated and applied to the injector. A threshold voltage higher than the normal voltage of the power supply is applied during a first threshold voltage impression period (e.g. 200 .mu.s) for this injection pulse. As a result, initiation of fuel injection by means of the injector is accelerated.
Thus, if the fuel injection timing for one cylinder is coincident with that for another cylinder, and then the first threshold voltage impression period for one injection pulse is coincident with that for the other cylinder, electric load increases considerably. In consequence, the voltage drops, and the performance characteristic of the injector and the injection quantity are lowered or reduced.
This situation may be eliminated by increasing the capacity of the power supply. Since the coincidence of the fuel injection timings is very rare, however, the increase of the supply capacity inevitably entails an increase in cost.
A measure to counter such coincidence of the fuel injection timings is described in Japanese Patent Laid-Open No. 63-212741 (1988). Fuel injection is effected in synchronism with the engine speed during a normal operation, and out of synchronism during acceleration. If asynchronous fuel injection is required during synchronous fuel injection, the timings for these injection operations may possibly coincide with each other. Usually, in this case, the coincidence is avoided by putting off the latter injection.
In the system disclosed in this publication, however, the fuel injection timing is changed in or out of synchronism with the engine speed. Thus, this system is quite different from a system in which the fuel injection timing is changed depending on the combustion mode, stratified or homogeneous. In the disclosed system, moreover, the latter injection is always deferred if the fuel injection timings are coincident, so that the injection is inevitably delayed by a large margin.